Mass production semiconductor devices are ordinarily tested before being sold. In this process, each and every device may be tested on specialized test equipments.
Semiconductor devices are manufactured in lots. Although the number is somewhat arbitrary, a lot could contain perhaps 1,000 semiconductor devices. According to known testing methods, each lot is tested by itself so as to keep the devices of the lot together. FIG. 1 is a schematic view of a known testing method. In FIG. 1, the first lot is labeled A11111.1. All devices of the first lot are loaded into the tester, which is represented by the shaded area, and the first lot devices are tested together. After testing is complete, the devices which failed are perhaps retested. Once finished with the first lot, the tester is unloaded. As can be seen, the same identification number, A11111.1, is used for the first lot after testing. The second lot, labeled A22222.1 is next loaded into the tester and tested in the same manner as was done for the first lot. Test equipment manufacturers are for instance Advantest and Teradyne. After the second lot is completed, a third lot is tested in a similar fashion.
As can be seen from FIG. 1, there are gaps between the first and second and second and third lots. These gaps relate to timing delays. During the delay, devices must be unloaded, test data must be downloaded from the test machine, the test machine must be reset and the next front end lot (“pre-test lot”) of devices must be loaded. The test methodology is similar to a conveyor belt which must be completely emptied before being reloaded. By analogy, the delay is similar to the delay between loading the first device.
The schematic shown in FIG. 1 provides lot integrity. After each stage of testing is complete, the operator can determine if the front end lot (or manufactured lot) experienced a problem during manufacture. For example, if the tests show that an unusually high percentage of the devices failed the test, then the conditions of manufacture can be reviewed and the lot can be specially treated such that a buyer does not unknowingly purchase devices which could have marginal quality. However, the delay associated with separately testing each manufactured lot is considerable.